Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable medical device, or in other non-medical devices as well.
An SCS system typically includes an Implantable Pulse Generator (IPG) 10 shown in plan and cross-sectional views in FIGS. 1A and 1B. The IPG 10 includes a biocompatible device case 30 that holds the circuitry and battery 36 necessary for the IPG to function. The IPG 10 is coupled to electrodes 16 via one or more electrode leads 14 that form an electrode array 12. The electrodes 16 are configured to contact a patient's tissue and are carried on a flexible body 18, which also houses the individual lead wires 20 coupled to each electrode 16. The lead wires 20 are also coupled to proximal contacts 22, which are insertable into lead connectors 24 fixed in a header 28 on the IPG 10, which header can comprise an epoxy for example. Once inserted, the proximal contacts 22 connect to header contacts 26, which are in turn coupled by feedthrough pins 34 through a case feedthrough 32 to circuitry within the case 30.
In the illustrated IPG 10, there are thirty-two lead electrodes (E1-E32) split between four leads 14, with the header 28 containing a 2×2 array of lead connectors 24. However, the number of leads and electrodes in an IPG is application specific and therefore can vary. In a SCS application, the electrode leads 14 are typically implanted proximate to the dura in a patient's spinal cord, and when a four-lead IPG 10 is used, these leads are usually split with two on each of the right and left sides of the dura. The proximal electrodes 22 are tunneled through the patient's tissue to a distant location such as the buttocks where the IPG case 30 is implanted, at which point they are coupled to the lead connectors 24. A four-lead IPG 10 can also be used for Deep Brain Stimulation (DBS) in another example. In other IPG examples designed for implantation directly at a site requiring stimulation, the IPG can be lead-less, having electrodes 16 instead appearing on the body of the IPG for contacting the patient's tissue.
As shown in the cross section of FIG. 1B, the IPG 10 includes a printed circuit board (PCB) 40. Electrically coupled to the PCB 40 are the battery 36 (which may rechargeable or permanent); other circuitry 50a and 50b coupled to top and bottom surfaces of the PCB (discussed further below with respect to FIG. 2); a telemetry coil 42 for wirelessly communicating with an external controller (not shown); a charging coil 44 for wirelessly receiving a magnetic charging field from an external charger (not shown) for recharging the battery 36 (if it is rechargeable) or for receiving continuous external power; and the feedthrough pins 34 (connection not shown). Further details concerning operation of the coils 42 and 44 and the external devices with which they communicate can be found in U.S. Patent Application Ser. No. 61/877,871, filed Sep. 13, 2013).
FIG. 2 shows the basic architecture of the circuitry within the IPG 10, and further details can be found in U.S. Patent Application Publication 2012/0095529, which is incorporated herein by reference. As shown, the IPG 10 includes a microcontroller 60 and one or more Application Specific Integrated Circuits (ASICs) 65 that communicate via a digital bus 75 and by off-bus signals. ASIC(s) 65 can include circuitry necessary for IPG 10 operation, including current generation circuitry (used to provide specified current pulses to selected ones of the electrodes 16); telemetry circuitry (for modulating and demodulating data associated with the telemetry coil 42); battery management circuitry (for controlling the connection of the battery 36 to the remaining circuitry, and/or to control its charging via charging coil 44); various measurement and generator circuits; system memory; etc. Off-chip components on the PCB 40 that would typically couple to the ASIC(s) 65 or the microcontroller 60, but which are not shown in FIG. 2 for convenience, include the battery 36; the charging coil 44; the telemetry coil 42; various DC-blocking capacitors coupled to the electrodes 16; and other components of lesser relevance here. Microcontroller 60 may comprise in one example Part Number MSP430, manufactured by Texas Instruments, which is described in data sheets at http://www.ti.com/lsds/ti/microcontroller/16-bit_msp430/overview.page? DCMP=MCU_other& HQS=msp430, which is incorporated herein by reference. The ASIC(s) 65 may be as described in the above-incorporated '529 Publication.
Also included off-chip is a thermistor 80, which can be used to detect the temperature of the IPG 10. The thermistor 80 is typically included in a resistive network, and in the simple example shown is coupled in series to a resistor R0, although other networks could be used with the thermistor. This series connection receives Va, which is a power supply voltage generated and regulated from the voltage provided by the battery 36 (such regulation not shown) and generally used to power analog circuitry in the IPG 10, which power supply Va may be a few Volts or so. As the resistance of thermistor 80 changes (e.g., decreases with increasing temperature), the voltage drop V0 across R0 changes (e.g., increases with increasing temperature), which voltage drop is reported to the microcontroller 60 at one of its Analog-to-Digital converter (A/D) inputs 61 to inform the microcontroller 60 of the IPG's temperature. Resistor R0 in the resistive network may also be adjustable or programmable, as discussed further below. Alternatively, the temperature as discerned from the thermistor 80's resistive network may be provided to the ASIC(s) 65, which may filter and buffer the analog signal, and provide it to the microcontroller 60 via an off-bus analog signal trace (not shown) for digitization.
It is useful to detect the temperature of the IPG 10 for many reasons. For example, once an IPG's manufacture is complete but before it is implanted in a patient, it may be necessary to verify that the IPG has not been subject to temperatures that are too hot or too cold. Exposure to extreme temperatures could occur for example when IPGs are being distributed to implanting clinicians. For example, if IPGs are shipped via airplane in a cargo hold that is not well temperature controlled, they may be exposed to temperatures that are too cold (e.g., <0 C). Or if shipped by truck for example, they may be exposed to temperatures that are too hot (e.g., >60 C). Monitoring for exposure to such extreme temperatures is important because such exposure can affect IPG quality and reliability. For example, the battery 36 included in the IPG 10, whether rechargeable or not, may become damaged at such extreme temperatures, even if such exposure is merely temporary. In fact, detecting of ambient temperatures during IPG distribution is a significant enough issue that temperature sensors external to the IPG can be included with their shipment, such as sensors placed on or in a box containing a number of IPGs being shipped. If IPGs are exposed to extreme temperatures, they may need to be returned to the manufacturer as unsuitable for implantation in a patient.
Another example in which detecting IPG 10 temperature is useful is during charging of the battery 36, assuming it is rechargeable, or otherwise when the IPG is receiving external power from an external charger. As is known, receipt of a magnetic charging field from an external charger can cause the IPG's temperature to increase, both by virtue of heating of the circuitry coupled to the charging coil 44 that receives and processes the received power, and by the induction of Eddy currents in conductive structures in the IPG 10, such as case 30. As explained in U.S. Patent Application Publication 2011/0087307, heating of the IPG 10 during charging can aggravate or damage a patient's tissue if a safe temperature is exceeded (e.g., >41 C). Thus, the microcontroller 60 can monitor whether thermistor 80 is reporting a temperature in excess of a safe threshold temperature, and can take appropriate action, such as by disabling the charging coil 44; disabling the generation of therapeutic stimulation pulses; disconnecting the battery 36 from the remainder of the IPG's circuitry, etc.
The inventors consider temperature sensing via thermistor 80 to have certain shortcomings. An IPG 10 may need to understand its temperature to an accuracy of +/−1 C, and over a relatively wide range of temperatures as discussed above. However, off-the-shelf discrete thermistors 80 may not be able to meet this desired level of accuracy. As such, it is typically necessary to calibrate the temperature reported by the thermistor 80. This adds complexity and time to the manufacturing process, and requires additional equipment. For example, the IPGs (preferably at an interim stage in which their circuitries are complete, but their batteries not yet attached) must “soak” at a known temperature (in an oven for example); the temperature of the thermistor 80 is then read by the microcontroller 60; with the microcontroller 60 then making an adjustment to align the reported temperature from the thermistor 80 with the known temperature. This adjustment to compensate for the thermistor 80's lack of accuracy can be internal to the microcontroller 60 (e.g., by altering its programming to converting the reported temperature to an accurate temperature), or by trimming the value of resistor R0. This calibration procedure preferably occurs at more than one temperature (e.g., at or near 0 C and 60 C, and perhaps at intermediate temperatures as well) to ensure proper calibration of the thermistor 80 over its intended operating range. Adding further difficulty to this procedure is that certain circuitry used in connection with reading the thermistor's temperature, such as the Analog-to-Digital Converter 61, may not function well as extreme temperatures, and may produce errors or add to the inaccuracy of the temperature measurement.
Additionally, the thermistor 80 is generally mounted to the IPG's PCB 40, which the inventors do not prefer. Although small, the thermistor 80 needs to be accommodated by the PCB 40, leaving less room for other components, and prohibiting reduction of PCB and IPG size. A surface-mounted thermistor 80 is also susceptible to mechanical damage.
Finally, typical surface-mounted thermistors 80 generally have resistances of 10 k-ohms or lower. The resistive network in which the thermistor 80 is included, which may include resistors of comparable resistance (e.g., R0), may therefore draw currents of at least tens of microAmps at typical levels for the analog-circuitry power supply voltage Va. This is a relatively significant current draw from power supply Va, and thus ultimately from the battery 36. This makes continuous temperature monitoring difficult, as temperature sensing will more quickly deplete a permanent battery 36, or require more frequent charging of a rechargeable battery 36.